Tree Hugger

The benefits of an urban forest in the concrete jungle

Any Boisean worth the appellation knows about Capt. Benjamin Bonneville, the Paris-born army officer, fur trapper and explorer whose party crested Boise's scrubby eastern hills in 1833, looked over the dry valley and saw a distant ribbon of green along the Boise River. "Les bois, les bois!" he shouted. "The trees, the trees!"

For Bonneville, who had been trekking with a company of soldiers from Missouri to Oregon, the Boise River was a welcome respite after weeks in the barren wilds of southern Idaho. His excitement was understandable.

"You've been to Pocatello. I'd be tickled, too," said Brian Jorgenson, Boise's city forester. "That whole stretch of Southern Idaho would be pretty bleak."

The Boise area was pretty bleak back then too, with the only trees growing along the water's edge.

What Bonneville couldn't have known was that 30 years later, a city named for his expedition's relieved exclamation would be founded near the riverbank, and that almost 130 years later, his oasis would form the roots of a sprawling forest comprised of about 290,000 trees.

Trees give Boise its name, provide its residents with shade and add grace and balance to its streetscapes. Jorgenson said they are also one of the community's greatest accomplishments.

"You climb up on the Foothills, and it's a forest. It's an urban forest," Jorgenson said. "And the vast, vast majority of what's in the valley is planted. What you look at when you look over the City of Trees is human-made."

The largest tree native to the valley—and the likely bois of the Bonneville party's happy cry—is the black cottonwood. Smaller trees like alders and serviceberry are also indigenous, but beyond those that grow naturally along the river, nearly everything else has been handpicked to take its place in the City of Trees.

And their benefits go far beyond aesthetics.

"The roots and the canopies make a natural stormwater filtration system. It would also be a lot hotter here, and trees do their part in keeping the air clean, too," said Jorgenson, whose Community Forestry Office operates under the Parks and Recreation Department. "For us, it's not about planting trees because they look pretty. We recognize all the benefits they have and try to bring as many trees in as we can."

If you've only ever hugged a tree for safety, hug one now in gratitude. According to the City of Boise Municipal Forest Resource Analysis, conducted in 2007 by the U.S. Forest Service, Boise's approximate 23,262 publicly managed street trees are among the city's hardest working citizens.

Each year, street trees intercept 19 million gallons of stormwater—a service worth more than $96,000 to the City of Boise. City-owned street trees also absorb and trap air pollutants like carbon dioxide, ozone and nitrogen dioxide. Researchers estimate that a full 900 tons of CO2 emissions are either pulled from the air or foregone because of photosynthesis each year, along with almost 7,000 pounds of ozone and 1,350 pounds of nitrogen dioxide. All that pollution control is worth a total of $6,292.

One of the largest benefits of Boise's trees comes in the form of energy savings. Shading and reduced temperatures from trees city wide each year decrease electricity use by more than 3,000 megawatt-hours and natural gas use by more than 129,230 therms. Total energy cost savings credited to street trees: $331,756 a year, an average of $14 per tree.

What's more, street trees contribute to the economy on their looks alone. According to the USFS study, homebuyers are far more likely to purchase a home with mature, attractive trees, and they're willing to pay more for the luxury. By comparing the difference in sales prices between treed and non-treed homes, researchers estimate that Boise's street trees contribute an average $24.16 per tree in aesthetic benefits.

On top of that, trees increase property values, attract businesses and draw shoppers to retail districts. A survey conducted by the University of Washington in 2005 found that consumers think treed shopping areas are better maintained, have higher quality products and more helpful merchants.

Survey respondents went even further, claiming they would be willing to pay 9 percent to 12 percent more for goods and services purchased in retail districts with large, well-cared-for trees. They even said they'd pay more for parking.

Taken as a whole, the City of Trees' publicly managed urban forest provides more than $1 million a year in economic benefits and costs a little less than $771,000 to maintain, making for a 30 percent return on investment.

Jorgenson said the 2007 USFS study only focused on city maintained trees. The benefits are likely higher when private trees are included in the forest inventory. According to the 2006 Boise Community Forestry Management Plan, there are more than 10,000 trees on the Greenbelt and river, and an additional 250,000 or so on private land.

"I don't know if there's ever been a quantification of how much of a role they play, but it's definitely a benefit," Jorgenson said. "We can make assumptions based on that."

Just counting city-owned trees, the USFS study concluded that pollution scrubbing in 2005—the year of the inventory—was equivalent to counteracting the carbon produced from more than 2 million driving miles.

Stephen Coe, Boise regional air quality manager for the Idaho Department of Environmental Quality, confirmed that there hasn't been a full study of the air quality effects from the city's trees, but "we all assume and know there are benefits."

DEQ monitors air quality at three sites in the Treasure Valley: White Pine Elementary School, on Linden Street; the Idaho Transportation Department headquarters, on State Street; and St. Luke's Meridian, on South Eagle Road.

Coe said the latest readings show the ITD site, which is closest to the Greenbelt, has the lowest ozone levels, with 63 parts per billion. The St. Luke's monitor registers 68 ppb and White Pine tops the list with 73 ppb.

Boise has long had trouble with its air quality, particularly during its wintertime inversions, when total air pollution levels are among their highest, but it stands to reason that without such a robust urban forest, those troubles would grow even larger.

"With the Greenbelt, specifically, the relationship to people commuting on bikes and things like that which would reduce vehicle miles, that's going to be a benefit," Coe said.

Though its effect on the airshed hasn't been studied specifically, the number of people the Greenbelt draws out of their cars and onto their bikes is substantial.

Amy Stahl, marketing and communications coordinator at the City of Boise Parks and Recreation Department, said a citywide survey showed 39 percent of residents use the Greenbelt one to five times a month. An additional 36 percent visit the 25-mile-long riverside path six or more times per month.

As a component of city planning, it's hard to go wrong with trees and parks, said Tricia Nilsson, Boise's comprehensive planning manager.

"From a streetscape to a park, the configuration of your vegetation can have a variety of micro-impacts throughout your city," Nilsson said. "There are pocket parks, shaded areas on the Grove—it improves the overall livability of the city."

Boise's greenscape is also one of its highest priorities, she added. As the city works on its new Comprehensive Plan, directives are being included to find and fill gaps in the urban canopy, promote tree-planting efforts and improve tree care and education efforts.

The economics of the forest are a plus, but Nilsson said the big dividends are paid in quality of life.

"The major theme is trying to promote a healthy, active lifestyle in Boise," she said. "We're the 'City of Trees,' and we do try to protect and encourage the growth and maintenance of our urban forest ... There is a public value that's recognized from the urban forest that's in our policy document."

Rachel Winer, executive director of Idaho Smart Growth, also said it's hard to overstate the benefits of urban trees and open space.

"It's about the community and providing people with a chance to be outside and in nature," she said. "They play a critical role [in planning]. We're not attracting people who want to live in concrete jungles—they're attracted to Idaho for that quality of life which is improved by those open spaces."

And Boise is rich in open spaces. Its 107 parks account for about 1,000 acres of open space, but the parks along the Boise River, often referred to as its "ribbon of jewels," are by far the largest. Ann Morrison Park sprawls across 153 acres, Julia Davis Park tops 89 acres, Kathryn Albertson Park is 41 acres and Municipal Park, to the east of the others, covers 28 acres.

According to the Parks and Rec survey, 64 percent of respondents said they visit Julia Davis Park as many as five times a month, and 63 percent reported going to Ann Morrison with the same frequency. As many as 66 percent of those surveyed reported living within walking distance of a park, and 34 percent said they could bike there.

All that helps the airshed as well. According to the Colorado Tree Coalition, urban parks like Boise's can reduce street-level particulates by up to 60 percent, and one acre of trees can store up to 2.6 tons of CO2 in a year—roughly equivalent to the amount of carbon dioxide produced yearly by each person in the United States.

The coalition also states that the cooling effect of trees goes a long way toward eliminating emissions. Through evaporation, trees decrease heat islands—areas where heat has become trapped in concrete, asphalt or other structures. Heat islands, or "heat sinks," can often be 19 degrees warmer than their surroundings, but the cooling effect of a single large tree is equivalent to 10 room-size air conditioners operating 24 hours per day—an added benefit during Boise's scorching summers.

"When it gets hot, a lot of times, one of the first places I go is to the park by the river, where there's trees and water, rather than getting into my car and running my air conditioning," Winer said. "It's an opportunity for us to enjoy clean air and clean water, and that's a great thing especially about the Boise Greenbelt, because so many neighborhoods and communities are close to it and have access to it—you don't to get into your car to get to it."

Maintaining that resource is "a science, along with everything else," Jorgenson said. While the average Boisean may not notice, the species of trees selected for various locations around the city are meticulously planned and monitored.

"It runs the gamut of what will grow in this area," Jorgenson said. "We try to keep a wide variety of tree species downtown so we're not looking at some nasty bug or disease down there wiping everything out."

The individual selections also have a bottom-line impact measured in environmental benefits—pollution control and energy efficiency—as well as aesthetic benefits like increased property values, business attraction and recreation.

According to the USFS analysis, Boise's 1,926 silver maples provide the highest annual payoff in environmental and aesthetic benefits, totaling almost $174,000 a year, or $90 a tree. American sycamore is the next most valuable, worth $87 per tree, and English elms are responsible for $85 per tree per year. Callery pear and hawthorn provide the next lowest level of benefit, at $19 per tree, and poor old crabapple is riding the poverty line at $17 per tree.

Tree-borne economic benefits are highest in the North End and West Boise along State Street, where the average is $50 a tree. On the Bench, trees average about $47 each, and the Greenbelt area downtown sees an average benefit of $45 per tree.

There are 77 species of trees in Julia Davis Park, though most are either maple or oak. Pine leads the pack at Ann Morrison, Jorgenson said, and Municipal Park's 225 trees are drawn from 10 species.

Trees along the Greenbelt are pretty much allowed to run riot, he added.

"We can't control, nor do we really want to control, the vegetation along the river," Jorgenson said. "The trees along there provide shade for fish, homes for wildlife, the river is cooler than it would be without the trees. The trees fall and leaves fall in the river. It provides organic matter for things to feed the fish, the mink, the beaver. Without those trees and plants, that ecosystem wouldn't exist."

It's a challenge to defend, however, with riverside real estate in high demand.

"Everybody wants to be close to the water and be a part of that natural scene," Jorgenson said. "We've definitely had our impacts ... It's not a wild river anymore. We've done our best to tame it."

Another challenge is vegetation's effect on the city's hardscapes—its concrete and asphalt surfaces are often the victim of straining roots. The highest single cost of maintaining Boise's urban forest is tree removal, which runs more than $141,000 a year, followed by administration, which costs about $124,500.

Because of their age and structure, silver maple trees account for the largest chunk of the maintenance expense, including removal, storm cleanup and property and infrastructure damage. In total, the city spends about $88,600 a year on repairs and liability due to tree damage.

Still, the benefit-cost ratio is rosy. USFS figures report that Boise's trees return $1.30 for every $1 spent on their management. That's similar to numbers reported by Berkeley, Calif.; Charleston, S.C.; and Albuquerque, N.M. Cities like Fort Collins, Colo.; Cheyenne, Wyo.; and Bismarck, N.D., get a higher return on their forests, but Jorgenson said that's simply because Boise isn't a naturally treed environment.

"We're lower on the benefits of street trees simply because we're in such a dry climate," he said. "We are in a desert."

And that's likely what Capt. Bonneville saw when he and his fellow explorers stood, exhausted, at edge of the valley: a parched, sage-dotted expanse unbroken but for the river and its copses of tall black cottonwoods.

"The river is really kind of the backbone of the urban forest," Jorgenson said. "It holds up the few remaining native trees in the area. It's still a huge part of our urban forest and the reason people want to live here. The river would not be what it is without the trees, and the trees wouldn't be there without the river."